So, the second alien ruler is fucking ridiculous. I'm going to try the mod that dteowner recommended above, otherwise I just don't see how. I like a good challenge, but these guys are unfair to the point of feeling like cheap design.
You'll hardly get to kill the Rulers the first time you encounter them (I couldn't) but if you do enough damage they'll flee to fight you again later - and they won't heal the damage you've caused.